The present invention relates to supplying power steering systems from a common pressure source and more specifically the application of a priority flow valve in a load sensing hydraulic system which provides priority flow to the steering circuit. In modern variable displacement hydraulic systems it quite often becomes necessary to provide an order of priority for the pump output capacity since the pump can not supply all of the circuits of the system with their maximum flow requirements at the same time.
For example, the power steering circuit of a mobile hydraulic system must have priority over the other circuits to avoid an obviously dangerous situation. In the past, most power steering systems have been supplied by a small independent pump which supplies only the needs of the steering system. However, in the interest of economics, some modern systems have taken the hydraulic pressure for steering from a large single pump which supplies all of the other hydraulic functions of the machine. A system of this type requires some form of priority flow to the steering unit. Two systems of this type are shown in U.S. Pat. No. 3,979,908 issued to Alderson, and U.S. Pat. No. 4,043,419 issued to Larson. Both of these patents teach priority flow control valves which are practically identical in function and structure. In both valves, to effect a movement of the valve spool, and adjust the priority flow, oil must backflow from the steering unit back to the servo chamber of the priority valve which quite often can be 15 to 20 feet. The response time of both of these valves is too slow due to a variety of reasons. Not only is there a long traveling distance, but the oil is cold and the size of orifice 52 in the Alderson patent and 57 in the Larson patent restrains its movement. If the orifice sizes in these two above-mentioned patents are increased, the stability of the valve decreases and the performance of the valve is unacceptable. In colder climates, the oil in this long sensing line remains very cold and thick since the oil in the line is basically stagnate.
Another disadvantage of the Alderson and Larson valves is the fact that spring 55 of Larson and 32 of Alderson must control the total pressure drop across the steering unit, therefore when the valves are standing by in a neutral condition, the priority valves will maintain a pump standby pressure which is fairly high and energyconsuming.